Epimutations are self-sustaining chromatin aberrations induced by transient exposures to environmental factors such as diet and smoking, with developing embryos particularly susceptible. Epimutations induced in the germ cells can be transmitted to the offspring, which has profound clinical, biological and evolutionary implications. Epimutations are far more common than classical DNA mutations, and underlie diverse human diseases. Despite their stability, epimutations are intrinsically reversible, which has fueled the concept of epigenetic therapy that seeks to repair epimutations using epigenetic drugs targeting relevant enzymes. At present, little is known about the mechanisms of induction and propagation of epimutations, and the epigenetic drugs are rather toxic. We have developed a novel animal model to address these problems. Specifically, we have modified the CD4 locus to allow for experimental manipulation of its epigenetic states; CD4 is an antigen coreceptor expressed on the surface of CD4 T cells but repressed in CD8 T cells. We found that transient activation of the CD4 locus in utero destabilizes CD4 repression in CD8 T cells in the ensuing adult mice and even their offspring, indicating that we have successfully induced an epimutation at the CD4 locus. Here we propose to use this system to address the mechanisms of the induction and transmission of an epimutation, taking advantage of the fact that CD4 regulation has been extensively studied. To this end, we will define the chromatin defects associated with the epimutation in CD8 cells, dissect the process leading to the establishment of the epimutation during fetal development, and characterize the chromatin defects in sperm (Am 1). In addition, we will test a novel strategy for selectively repairing the epimutation at the CD4 locus (Aim 2). Our study will provide long-awaited insights into epimutations, and establish a new paradigm for studying and repairing epimutations. PUBLIC HEALTH RELEVANCE: Adverse environmental conditions can stably changes gene functions without mutating DNA, and such changes underlie many human diseases. We will study the mechanisms of this remarkable phenomenon and seek to reverse the aberrant changes using a new approach.)